1. Field of the Invention
The present invention relates to an electromagnetic wave absorber that is preferably used for an electromagnetic wave anechoic room or the like and a method of manufacturing the electromagnetic wave absorber, and to an electromagnetic wave anechoic room.
2. Description of the Prior Art
An electromagnetic wave anechoic room for EMC (Electromagnetic Compatibility) has been widely put into practical use as an examination site for measuring electromagnetic wave noise radiated from various types of electronic equipment and evaluating tolerance of electronic equipment interfered by external electromagnetic wave noise. Also, in recent years, there is a movement that the electromagnetic wave anechoic room is used for a place (CALTS=Calibration Test Site) to calibrating an antenna for radiation noise measurement.
Electromagnetic wave absorbers are installed on a ceiling and walls of an electromagnetic wave anechoic room for EMC to thereby provide a space where electromagnetic wave reflections from any positions other than a floor surface (metal surface) can be minimized. As the electromagnetic wave absorber to be used on the ceiling and walls of the electromagnetic wave anechoic room for EMC, a complex type electromagnetic wave absorber is currently employed in many cases. The complex type electromagnetic wave absorber is, as shown in FIG. 20, a combination of a sintered ferrite compact 1 as an electromagnetic wave absorption member consisting of magnetic loss material and an electromagnetic wave absorber 2 containing electrically conductive material.
As the electromagnetic absorber containing electrically conductive material, an absorber having a pyramid or wedge shape has conventionally been often employed, wherein a base material (a dielectric material having a low dielectric constant) such as foamed polystyrol or foamed polyurethane is used to retain electrically conductive material such as carbon or graphite. The length of the electromagnetic wave absorber is generally approximately 0.5 to 2 m, and a larger and higher-performance electromagnetic wave anechoic room requires a longer absorber. For this reason, problems exist that the electromagnetic wave absorber becomes voluminous, thus causing the increases in transportation and installation costs.
In consideration of the above problems, an electromagnetic absorber has been proposed in order to reduce the cost by reduction of materials necessary, transport volume, weight, and difficulty in installation, wherein the above-described electromagnetic wave absorber is modified to have a hollow structure consisted of sheet-type electromagnetic wave absorption members containing electrically conductive material, thus enabling it to be transported in such a sheet-type condition and then assembled on a site.
Such a hollow-structured electromagnetic wave absorber includes the hollow pyramid type shown in FIGS. 21A and 21B, or the hollow wedge type shown in FIGS. 22A and 22B, or FIGS. 23A and 23B. In FIGS. 21A, 21B, 22A, 22B and 23A, 23B the reference numeral 1 represents a sintered ferrite compact, and 2 a hollow electromagnetic wave absorber that contains electrically conductive material and is arranged in front of the sintered ferrite compacts. The hollow wedge type shown in FIGS. 23A and 23B has an open face on the side (triangular face).
Japanese Patent Application Laid-Open Nos. 11-87978 and 2000-216584 describe examples of publicly known technologies for a hollow electromagnetic wave absorber containing electrically conductive material.
Also, as described in Japanese Patent Laid-Open Nos. 2-97096 and 2001-127483, a rectangular pipe-shaped electromagnetic wave absorber and an electromagnetic wave absorber in which the electromagnetic wave absorbing plates are crisscrossed have also been disclosed.
Meanwhile, the wedge type electromagnetic wave absorber is anisotropic for a polarization plane of an incoming electromagnetic wave and therefore exhibits different characteristics depending on the polarization plane of the incoming electromagnetic wave. In particular, in the case of the hollow wedge type comprised of sheet-type electromagnetic absorption members, there is the problem that the difference in the characteristics caused by the polarization plane is significantly large and high-frequency characteristics are extremely poor when the ridge of the wedge is perpendicular to the polarization plane of the electromagnetic wave. In order to solve this problem of difference in the characteristics caused by the polarization plane, there is a method in which the neighboring absorbers are arranged in such a way that the ridges of wedges become orthogonal to each other when installed on wall surfaces, thereby averaging out the characteristics in the case where the ridge of the wedge is parallel to the polarization plane and in the case where the ridge is perpendicular to the plane. However, with high frequencies, the characteristics in one of the cases (the case where the ridge of the wedge is perpendicular to the polarization plane of the electromagnetic wave) are extremely poor, and therefore the average characteristics also become poor. Furthermore, in the case where the absorbers are arranged on sidewall surfaces as described above, the electromagnetic absorbers arranged in such a way that ridges of the wedges of them are horizontal are half of the total in number, and in such an arrangement, if the length is increased, a problem arises in terms of strength, such as bending. These problems are more significant, particularly in the case of the side face opening type shown in FIGS. 23A and 23B that is more advantageous in cost, productivity, and installation.
On the other hand, the hollow pyramid type is often used since it has no difference in characteristics caused by a polarization plane and is also robust in terms of strength; however, it has poor low frequency characteristics in a range of 30 to 100 MHz in comparison with the hollow wedge type, whereby there is a problem that a length of the absorber should be increased.
In consideration of this problem, a configuration provided with an opening at a tip of a hollow cone-shaped body has been proposed in Japanese Patent Application Laid-Open No. 2005-340730 by the present assignee as an electromagnetic wave absorber having no difference in characteristics caused by the polarization plane and good low frequency characteristics in the 30 to 100 MHz range.
However, there is a problem in the electromagnetic wave absorber having the configuration provided with an opening at a tip of a hollow cone-shaped body disclosed in Japanese Patent Application Laid-Open No. 2005-340730. The problem is that although an electromagnetic wave can reach sintered ferrite compacts through the opening at higher frequencies, an absorption capability of the sintered ferrite compact is low at a high frequency of 1 GHz or higher, and so reflection becomes large. Therefore, an additional electromagnetic wave absorber needs to be added on the bottom in order to improve high frequency characteristics, so that the advantage of being configured in sheet-type cannot be sufficiently utilized.
Similarly, the rectangular pipe-shaped electromagnetic wave absorber disclosed in Japanese Patent Application Laid-Open No. 2-97096 and the electromagnetic wave absorber in which electromagnetic wave absorbing plates are crisscrossed disclosed in Japanese Patent Application Laid-Open No. 2001-127483 have also a problem of poor high frequency characteristics because of the exposure of a sintered ferrite compact. In order to improve the high frequency characteristics, the opening of the rectangular pipe-shaped should be narrowed or a small electromagnetic wave absorber needs to be added on the bottom of the crisscrossed electromagnetic wave absorbing plates, so that the advantage of being configured in sheet-type cannot be sufficiently utilized in either case as well.